A process called chemical mechanical polishing (CMP) is commonly carried out for polishing the surface of a semiconductor device, display panel e.g. of LCD, lens, hard disk substrate, metal, etc., using various polishing powders composed mostly of metal oxides dispersed in water or an organic solvent with a proper abrasive pad. Such metal oxides used in CMP include silica(SiO2), alumina(Al2O3), ceria(CeO2), zirconia(ZrO2), tin oxide(SnO2) and manganese oxide(MnO2), and a variety of polishing powders have been continuously developed in response to the increasing demand by precision electronic industries for high precision polishing.
Two important performance parameters in CMP are the polishing speed and the quality of polished surface, i.e., absence of scratches, and these parameters depend primarily on the size and shape of polishing particles.
As the size of particles increases, the polishing speed increases but creates more scratches. Therefore, the size of polishing particles is controlled by balancing the required quality of the polished surface and the polishing speed. Generally, the maximum size of polishing particles for high precision polishing is 1 μm or less. For polishing semiconductor devices and display panels which require utmost high precision, submicron to nanometer hyperfine polishing agents have been used to minimize the occurrence of scratches.
As to the shape of polishing agents, it is preferred to make the shape of particles irregular so that the contact area between the particle surface and the substrate surface can be maximized. By making the shape of particles irregular without increasing the particle size, the polishing speed can be enhanced without producing scratches. The irregular particle shape is formed by aggregating primary particles smaller than the desired particles size through relatively weak cohesive interaction. Excessively strong cohesiveness of such aggregates lead to increased scratches.
Conventional techniques for preparing metal oxide nano particles used in polishing are based on precipitation, gaseous synthesis and milling, and such techniques generally produce a significant amount of massive aggregates of smaller particles which must be removed.
To avoid such a problem, PCT International publication No. WO 99/59754 (The University of Western Australia) discloses a spherical polishing agent produced by adding a diluent such as NaCl, CaCl2, MgCl2, Na2SO4, Na2CO3, Ca(OH)2, CaO, MgO to a metal compound and milling to prevent the metal compound particles from agglomerating. Such diluents do not react with metal compounds and can be easily removed by washing with a solvent. However, the metal oxide particles produced by this procedure are not of desirable irregular shape for providing a high polishing speed. This is considered to arise from the fact that the concentration of the diluent used is excessively high, 80 vol % or more based on the total volume of the metal oxide-diluent mixture, which leads to non-aggregated, highly dispersed nano particles.
Accordingly, the present inventors have endeavored to develop an improved metal oxide powder- for polishing and found that aggregates of primary spherical particles having irregular particle shape can be produced by a carefully controlled procedure and used for high-speed, high-precision polishing.